In general, a magnetoresistive (MR) element is an element which utilizes a phenomenon in which the application of magnetic flux to a thin film pattern of an iron-nickel alloy, a cobalt-nickel alloy, or the like changes its magnetoresistance. Position detecting devices incorporating this kind of magnetoresistive elements are widely used in conjunction with a magnetic recording medium (e.g., ferrite or plastic magnet) for enabling detection of the position of the magnetic recording medium. Specifically, a sinusoidal reproduction output can be obtained by utilizing the change in the magnetic flux due to the movement of the magnetic recording medium. By processing the output waveform, the relative or absolute position of the magnetic recording medium is obtained with a high accuracy. This kind of position detecting device is disclosed in Japanese Laid-open Publication No. 1-203922, for example, and a position detecting device which has a configuration as shown in FIG. 12 is widely implemented in apparatuses for consumer or industrial use.
FIG. 13 is a perspective view showing the general structure of a position detecting device incorporating a magnetoresistive element, and FIG. 14 is its front view. On the surface of a magnetic recording medium 201 is provided a magnetic pattern 202, which in magnetized so as to have N poles and S poles with a predetermined period .lambda.. It has a structure such that a holder 204, which is integrally formed with magnetoresistive elements, is placed so as to oppose the magnetic recording medium 201 with predetermined spacing therefrom.
The operation principle of magnetoresistive elements is now explained with reference to FIGS. 15A through 15E. The change in the resistance of a magnetoresistive element in response to magnetic flux has the characteristics an shown in FIG. 15A, such that the resistance changes in proportion to the magnitude of the magnetic flux regardless the direction of the magnetic flux, and reaches saturation at a certain value. On a sensor face 205 of a holder 204, two magnetoresistive units R1 and R2 are disposed with an interval .lambda./2, which is equal to half of the period .lambda. of the magnetic pattern 202, or an electrical angle of 180.degree. along the direction of movement of the magnetic recording medium 201.
Now, a case is considered where the magnetic recording medium 201 moves, and magnetic flux B whose magnitude changes in sinusoidal waves as shown in FIG. 15B is applied to the magnetoresistive units R1 and R2. If such magnetic flux B is applied to the magnetoresistive units R1 and R2, the resistance values of the magnetoresistive units R1 and R2 vary with the period .lambda., with a phase difference of .lambda./2 as shown in FIG. 15C.
Therefore, as shown in FIG. 15D, if these magnetorsistive units R1 and R2 are serially connected, and a voltage V in applied from a DC power supply 210, an output E1 can be obtained at a connection point 211. As shown in FIG. 15E, the output E1 is a sine wave output having the period .lambda..
Now, as understood from FIGS. 15A, 15B, and 15E, the amplitude of the sine wave output E1 increases or decreases corresponding to the amplitude of the magnetic flux B. This means that, if the spacing between the sensor face 205 and the magnetic recording medium 201 becomes wider, the amplitude of the magnetic flux B, which changes in accordance with the motion of the magnetic recording medium 201, becomes smaller, so that the sine wave output E1 also becomes smaller. In order to detect the position of the magnetic recording medium by processing the sine wave output E1, a high signal-to-noise ratio is required. Thus, it is necessary to increase the amplitude of the sine wave output E1. Therefore, it is necessary to decrease the distance between the sensor face 205 and the magnetic recording medium 201 so as to increase the amplitude of magnetic flux B.
At the same time, as seen from FIG. 15A, the resistance change of a magnetoresistive element saturates at a certain value. If the amplitude of the magnetic flux B is too large, the resistance of the magnetoresistive element reaches saturation. Therefore, the amplitude of sine wave output E1 can only increase so much. On the contrary, the saturation of the resistance change amount gives rise to an expanse of areas in which the resistance remains unchanged despite changes in the magnetic flux and the output E1 is distorted.
As is understood from the above, it is necessary to adjust the distance between the sensor face 205 and the magnetic recording medium 201 to a predetermined distance known as a reference gap amount in order to increase the amplitude of the sine wave output E1 while preventing distortion of the sine wave output E1.
The foregoing is a description of the principle of magnetic flux change detection. Now, a method for determining the moving direction of magnetic recording medium 201 will be explained, employing, four magnetoresistive units R1, R2, R3, and R4 shown in FIG. 16. The magnetoresistive units R3 and R4 are disposed with an interval of .lambda./2 along the moving direction of the magnetic recording medium 201, in a manner similar to the magnetoresistive units R1 and R2.
A pair of magnetoresistive units R3 and R4 are disposed with an interval of 1/4.lambda., i.e., an electrical angle of 90.degree., with regard to the pair of magnetoresistive units R1 and R2, and electrically connected as shown in FIG. 17. Then, if a voltage V is applied from a DC power supply 210, a phase-A output Ea is obtained at an output terminal 212, and a phase-B output Eb is obtained at an output terminal 213. As shown in FIG. 18, the phase-A output Ea and the phase-B output Eb are shifted from each other by an electrical angle of 90.degree. (1/4.lambda.), so that their phases advance differently depending on whether the moving direction of magnetic recording medium 201 is positive or negative. Based on this, it is possible to determine the moving direction of the magnetic recording medium 201.
On the other hand, the amount of resistance change of a magnetoresistive element is as small as 2%. In an actual position detecting device, it is commonplace to dispose a plurality of the same phase magnetoresistive elements with the distance .lambda. in order to increase the amount of resistance change. That is, as shown in FIG. 19, eight magnetoresistive units R11, R12, R21, R22, R31, R32, R41, and R42 are used. Here, the magnetoresistive units R11 and R12 are disposed with the distance .lambda. along the moving direction of the magnetic recording medium 201, so as to be equivalent to the magnetoresistive unit R1 shown in FIG. 17. The pair of magnetoresistive units R11 and R12 and the pair of magnetoresistive units R21 and R22 are disposed with an interval of .lambda./2 so as to be equivalent to the magnetoresistive units R1 from R2 shown in FIG. 17.
The four magnetoresistive elements, i.e., the magnetoresistive units R11, R12, R21, and R22 and the magnetoresistive units R31, R32, R41, and R42 are disposed with an interval of 1/4.lambda.. The magnetoresistive elements disposed in such a pattern are equivalent to the electrical circuit of FIG. 17, and yet twice as much magnetoresistance change is obtained.
Among lens barrels used in cameras or video cameras and the like, a barrel is known in which the lens is moved by a linear motor when zooming or focusing. When moving the lens using such a motor, a separate position detecting means is required because the motor itself does not have position information. Therefore, methods are known which employ a position detecting device incorporating the aforementioned magnetoresistive element for detecting the position of a lens.
FIG. 20 shows a lens barrel structure employing such a linear motor. On an open face 120 located forward of a rear lens barrel 103 along the optical axis direction, a fixed lens frame 122 holding a compensation lens array 121, which is an array of fixed lenses, is attached, and a zooming lens array and a front lens barrel (not shown) are further disposed in this order along the optical axis direction.
Inside the rear lens barrel 103, a focusing lens 102 is held by a lens frame 101. The lens frame 101 is supported, by guide shafts 104a and 104b whose ends are affixed to the rear lens barrel 103 and the fixed lens frame 122, so as to be slidable along the optical axis direction (Z-axis direction).
The linear motor, which drives the lens frame 101 along the optical axis direction, includes the following stators: a driving magnet 105 which is magnetized perpendicularly to the moving direction (Z-axis direction), a C-shaped main yoke 106, and panel-like side yoke 107 provided on the rear lens barrel 103. The linear motor also includes, as moving portion, a coil 109 which is fixed on the lens frame 101 with a certain gap away from the driving magnet 105. When an electric current is applied to the coil 109, in a direction perpendicular to the magnetic flux generated by the driving magnet 105, the lens frame 101 is driven along the optical axis.
Next, a position detecting device will be described. FIG. 21 is a cross-sectional view taken along the line A--A in FIG. 20. The lens frame 101 includes a magnetic scale 111 formed of a magnetic recording medium such as ferrite. The surface of the magnetic scale 111 is alternately magnetized so as to have S poles and N poles with a pitch of 150 to 400 .mu.m along the given optical axis direction (Z-axis direction), which is identical with the driving direction of the lens frame 101.
Moreover, the holder 112 in the position detecting device shown in FIG. 12 is held by the rear lens barrel 103, and a sensor face 113 which is formed of a magnetoresistive element opposes the magnetic scale 111 at a certain distance therefrom. When a pin 114 is inserted into a pivoting hole 117 (FIG. 20), the holder 112 can pivot around it. Therefore, a method is commonly used in which the spacing between the magnetic scale 111 and the sensor face 113 is adjusted by pivoting the holder 112, and thereafter the holder 112 is fixed by means of a screw 116 which is inserted into an elongated aperture 115 (FIG. 20).
However, in a position detecting device having the aforementioned structure, the step of adjusting the spacing between the magnetic scale 111 and the sensor face 113 and the step of fixing the holder 112 should be separately performed, thereby complicating the assembly of the apparatus, and thus causing an increase in the manufacturing cost.
Moreover, as described with reference to FIG. 15A, the spacing between the magnetic scale 111 and the sensor face 113 should be set at a predetermined distance which is known an the reference gap amount. As the spacing becomes wider than the reference gap amount, the output of the position detecting device drastically decreases; on the other hand, as the spacing becomes narrower, the output is distorted so that a sinusoidal reproduction output can no longer be obtained.
When the magnetization pitch of the magnetic scale 111 is 200 .mu.m, for example, the reference gap amount should be set at around 100 .mu.m. In this case, in order to enable highly accurate position detection based on the output of the position detecting device, the spacing between them should be set, for example, within about .+-.20 .mu.m tolerance. The step of manually pivoting the holder 112 around the pin 114 and positioning the holder 112 within such a small tolerance is extremely difficult, and this has been causing a further increase in the manufacturing cost.
Furthermore, given the processing accuracy, there is a gap of 10 .mu.m or more between the pin 114 and the pivoting hole 117 in which the pin 114 is inserted. Thus, when the holder 112 is intended to be pivoted around the pin 114, the holder 112 may often be translated along the gap with respect to the pivoting hole 117. Therefore, it has been difficult to make a fine adjustment on the order of 10 .mu.m.
The following methods are known to provide a constant gap between a magnetoresistive element and a magnetic recording medium without performing such an adjustment.
A position detecting device disclosed in Japanese Laid-open Publication No. 62-157522 is characterized by using a thin flexible film for setting the spacing between a magnetic recording medium and a magnetoresistive element. A position detecting device disclosed in Japanese Utility Model Laid-open Publication No. 2-97617 is configured so that a magnetoresistive element is fitted in a resin or metal holder, and the spacing between a magnetoresistive element and a magnetic recording medium is set by allowing a projection extruding from the holder to abut the magnetic recording medium.
In these conventional technique, the spacing between magnetoresistive element and the magnetic recording medium is physically bridged through a thin film or a projection, so that, the spacing between the magnetic recording medium and the magnetoresistive element can be set in the vicinity of a certain width by simply attaching the magnetoresistive element.
In these conventional techniques, however, the output of the driving means for driving the moving portion should be enhanced because the moving portion must move while maintaining physical contact. Particularly in the case of a lens barrel, where a light-weight moving portion having a weight of only 1 to 2 g is driven, the friction force generated due to the physical contact of the position detecting device becomes extremely large compared to the weight of the moving portion. In order to move the moving portion against such friction force, the linear motor for driving it must be increased in size. As a result, there has been a problem in that only a lens barrel having a quite large overall size can be provided.
Moreover, in the conventional example described with reference to FIG. 20, where the holder 112 is affixed by the screw 116 which is inserted into the elongated aperture 115, there is a disadvantage in that the holder 112 has such a large configuration that the area occupied by the holder 112 along the width direction (X-axis direction) of the barrel increases, thereby hindering downsizing of the apparatus.
Furthermore, the magnetic scale 111 and the sensor face 113 are not always in parallel because the holder 112 is pivoted around the pin 114, so that the spacing between them varies along the optical axis direction (Z-axis direction). If the spacing between the magnetic scale 111 and the sensor face 113 varies along the optical axis direction (i.e. the moving direction of magnetic scale 111) in this manner, there is a disadvantage in that the output characteristics of the position detecting device deteriorate, as described below.
Usually, magnetoresistive elements consist of eight magnetoresistive units R11 through R42, as shown in FIG. 19. In addition, the magnetoresistive units R11 through R42 should be disposed in a certain pattern so as to be at certain distances from each other, along the moving direction of the magnetic recording medium 201 (i.e., the magnetic scale 111). The sensor face 113 of the holder 112 shown in FIG. 12 is also provided with similar magnetoresistitve units R11 through R42.
As is understood from FIG. 19, the distance from the magnetoresistive units R11 to R42 is 2.75 times the magnetization period .lambda. of the magnetic scale 111. In the case where .lambda.=200 .mu.m, which exemplifies a typical magnetization period of the magnetic scale 111, the distance from the magnetoresistive units R11 to R42 is 550 .mu.m.
Suppose the sensor face 113 has tilted by .theta.=5.degree. with respect to the magnetic scale 111 around the Y-axis as a result of an adjustment by pivoting the holder 112 around the pin 114, as shown in FIG. 22. In this case, there is as much as a 50 .mu.m difference between the distance S1 from the magnetoresistive unit R11 to the magnetic scale 111 and the distance S4 from the magnetoresistive unit R42 to the magnetic scale 111. When the magnetization pitch of the magnetic scale 111 is 200 .mu.m, the reference gap amount should be set at around 100 .mu.m. Therefore, the spacing for the magnetoresistive unit R11 differs from the spacing for the magnetoresistive unit R42 by nearly half the reference gap amount.
As described with reference to FIG. 15A, the amount of resistance change of the magnetoresistive element varies depending on the magnitude of magnetic flux, in other words, the spacing between the magnetic scale 111 and the magnetoresistive units R11 through R42. In the vicinity of the portion of the magnetoresistive unit R11, the spacing is narrower than the reference gap amount, so the resistance change of R11 is distorted. On the other hand, in the vicinity of the magnetoresistive unit R42, the spacing is wider than the reference gap amount, so the amplitude of the resistance change is small.
If the spacing between the magnetoresistive elements and the magnetic scale 111 varies along the moving direction of magnetic scale 111 in this manner, the amount of resistance change varies for each of the magnetoresistive elements which are disposed in a certain pattern and at a certain distance to each other. Therefore, even if the spacing is adjusted equal to the reference gap amount in the center, the A-phase output Ea and the B-phase output Eb shown in FIGS. 17 and 18 may have distorted waveforms, or the amplitudes of A-phase output Ea and B-phase output Eb may be different, or the phase difference between them (1/4.lambda.) may vary.
The resolution and accuracy of position detecting devices are becoming increasingly important in recent years. For example, as for lens barrels used for cameras or video cameras, etc., the trend for downsizing, lighter weight, and higher performances of the products give rise to the need for developing smaller lens barrels having better optical characteristics. In order to downsize a lens barrel or improve the optical characteristics (such as resolution) of a lens, it is necessary to position the lens with a higher accuracy upon zooming or focusing. Therefore, the resolution and accuracy of the position detecting device need to be enhanced correspondingly. Therefore, there is a need for obtaining an accuracy of about 1 .mu.m, which is much smaller than the magnetization period .lambda.(=200 .mu.m)of the magnetic scale 111, by subjecting the A-phase output Ea and B-phase output Eb to complicated processing.
In order to secure such a high position detection accuracy, the waveform accuracy of A-phase output Ea and B-phase output Eb becomes quite important. That is, if the waveforms are distorted, or they differ in amplitude, or the phase difference between these outputs varies, the accuracy of the position detection is decreased.
In the position detecting device having the configuration described with reference to FIGS. 12, 20 and 21, the sensor face 113 may tilt with respect to the magnetic scale 111 as shown in FIG. 22 at the time of adjusting the spacing between magnetic scale 111 and sensor face 113. Therefore, there has been a problem in that the spacing between the magnetic scale 111 and the sensor face 113 varies along the moving direction of magnetic scale 111, and the accuracy of the waveforms at A-phase output Ea and B-phase output Eb is deteriorated, thereby making it impossible to secure high resolution and accuracy.